Allow assignment to numeric string via assertion

src/Psalm/Type/Atomic.php

@@ -110,6 +110,9 @@ abstract class Atomic
             case 'class-string':
                 return new TClassString();
 
+            case 'numeric-string':
+                return new TNumericString();
+
             case '$this':
                 return new TNamedObject('static');
 

tests/TypeReconciliationTest.php

@@ -775,6 +775,17 @@ class TypeReconciliationTest extends TestCase
                       return false;
                     }',
             ],
+            'numericStringAssertion' => [
+                '<?php
+                    /**
+                     * @param mixed $a
+                     */
+                    function foo($a, string $b) : void {
+                        if (is_numeric($b) && $a === $b) {
+                            echo $a;
+                        }
+                    }'
+            ],
         ];
     }